Water-soluble polymers (also commonly referred to as "thickeners" or "rheology modifiers") are widely used in many industrial water-borne systems as additives to modify their flow behavior. Thickeners increase and maintain viscosity at required levels under specified processing conditions and end use situations. Thickeners are useful, for example, in decorative and protective coatings, paper coatings, cosmetics and personal care items, detergents, pharmaceuticals, adhesives and sealants, agricultural formulations, and petroleum drilling fluids.
One such highly filled aqueous system where the thickener is used in a decorative and protective coating is latex paint, which is composed of a dispersion of a polymeric latex, pigments, clays, and other additives in water.
Thickeners can be materials that are either naturally occurring or synthetically manufactured. Natural thickeners, for example, include casein, alginates, gum tragacanth, guar, xanthan gum, locust bean gum, and modified celluloses, such as hydroxyethylcellulose, hydroxypropylcellulose, and carboxymethylcellulose. These natural products vary in their thickening efficiency. One of the drawbacks of natural product based thickeners is that they are subject to microbial attack and hence, addition of antimicrobial agents to the formulation is required. Synthetic thickeners include various acrylic polymers, alkylene oxide polymers, amide polymers, and maleic anhydride polymers. These synthetic thickeners can be either homopolymers or copolymers. The hydrolytic stability of some of these polymers depends on the pH of the solution and others are sensitive to various components normally found in aqueous coatings.
Regardless of whether the thickener is natural or synthetic, it is desirable that it be water-soluble to have optimal properties as a thickener in various water-based systems. The term "water-soluble polymer" refers to any polymeric substance which is soluble in water and, by being soluble, produces a change in solution viscosity, refractive index, or surface tension of water.
Typically, a small amount (about 0.1-5 wt. %) of water-soluble polymers is added to latex paints to achieve the following performance characteristics during manufacturing, storage, and applications:
a) Ease of formulation, PA1 b) Prevention of settling of the suspended particles (latex, pigment, etc.) during storage, PA1 c) Good film build during applications to achieve efficient hiding without excessive brush or roller drag, PA1 d) Good roller spatter resistance, PA1 e) No excessive sagging after application on a vertical surface, and PA1 f) Good flow and leveling for the formation of a smooth and continuous film with good appearance. PA1 a) reacting an alpha, omega-dihydroxy polyether with a gem-dihalide compound in the presence of a base to form an alpha, omega-dihydroxy poly[acetal- or ketal-polyether] backbone, and PA1 b) reacting the backbone with a hydrophobic reagent to form the hydrophobically end-capped poly(acetal- or ketal-polyether).
The above mentioned natural and synthetic thickeners provide different degrees of thickening efficiency and application properties. However, they invariably fail to provide all of the key performance properties in gloss paints. These key properties include good film build, flow and leveling, and gloss which are generally offered by solvent-based alkyd paints. Another drawback of these thickeners is that they may have poor compatibility with the various paint ingredients.
To eliminate some of the performance deficiencies of conventional thickeners, a new class of thickeners, commonly referred to as."associative thickeners", has been designed and commercialized (See E. J. Schaller and P. R. Sperry, in "Handbook of Coatings Additives", Ed. L. J. Calbo, Vol. 2, p: 105, 1992; Marcel Dekker, Inc., New York). These thickeners are hydrophobically-modified water-soluble polymers. They undergo intermolecular association in aqueous solution and thereby exhibit enhanced solution viscosity. They can also adsorb onto the dispersed-phase particles of an aqueous dispersion and thereby form a three-dimensional network. Since they provide improved paint properties not offered by conventional thickeners, they have gained commercial importance.
Polysaccharide based associative thickeners are made by chemically grafting a small amount of a hydrophobic group (C.sub.10 -C.sub.24 alkyl) onto a modified polysaccharide. They are disclosed in U.S. Pat. Nos. 4,228,277, 4,243,802, and EP 281,360.
Among commercial nonionic synthetic associative thickeners, hydrophobically modified ethoxylated urethane (HEUR) block copolymers constitute an important class. They are disclosed in U.S. Pat. Nos. 4,079,028, 4,155,892, and 5,281,654. These are low molecular weight polyurethanes made by condensing relatively low molecular weight polyethylene glycol (molecular weight.about.10,000) with hydrophobic diisocyanates and end-capping the resulting copolymer with hydrophobic alcohols or amines. They are characterized by having three or more hydrophobes--two of which are terminal and the remainder are internal. The hydrophobic groups are connected to the hydrophilic polyethylene oxide blocks through-urethane linkages.
The preparation of HEURs is also disclosed in U.S. Pat. Nos. 4,499,233 and 5,023,309. These HEURs are claimed to provide superior viscosifying properties and improved flow and leveling in aqueous systems.
Processes for the production of HEURs with pendant hydrophobes in bunches are described in U.S. Pat. Nos. 4,426,485 and 4,496,708. These HEURs are believed to lo provide enhanced thickening to aqueous systems through micelle-like association.
HEURs with branched structure and terminal hydrophobes are disclosed in U.S. Pat. No. 4,327,008. They are made by reacting polyalkylene oxides with a polyfunctional material, a diisocyanate, and water and end-capping the resulting product with a hydrophobic monofunctional active hydrogen-containing compound or a monoisocyanate.
Silicon-containing HEURs having particular utility in paints and coatings are disclosed in European Patent Application 0 498,442 A1. These are reaction products of an isocyanate functional material, a polyether polyol, a monofunctional active hydrogen-containing hydrophobic compound, a silane-functional material, and water.
From applications and economic point of view, the major drawbacks of HEUR thickeners are their high cost, difficulty in handling, and tendency to destabilize the paint (separation of dispersed particles from the aqueous phase) (G. D. Shay and A. F. Rich, J. Coatings Technology, Vol. 58, No. 7, p.43, 1986).
Random copolymers of ethylene oxide and long chain alkyl epoxides are disclosed in U.S. Pat. No. 4,304,902. These copolymers provide enhanced aqueous viscosity, but do not provide good flow and leveling in latex paints.
U.S. Pat. No. 4,411,819 describes the preparation of polyethers which have branched chain structure and are characterized by having terminal hydrophobes. They are made by reacting a low molecular weight polyol with a mixture of ethylene oxide and at least one lower alkylene oxide having 3-4 carbon atoms. The polyethers are then end-capped with a mixture of C.sub.12 -C.sub.18 alpha-olefin epoxides.
Low molecular weight (.about.9,000) hydrophobically end-capped polyether is disclosed in PCT Int. Appl. WO 92 08753. These are made by coupling low molecular weight (.about.4,500) surfactant with m-dichloromethylbenzene. Preparation of low molecular weight (.about.9,000) hydrophobically end-capped polyethers is also disclosed in U.S. Pat. No. 5,045,230. These are made by reacting C.sub.8 -C.sub.22 aliphatic alcohols with a mixture of ethylene oxide and propylene oxide and subsequently coupling the alkoxylated alcohols with a diepoxide to form a polyether (MW.about.9,000). Since these hydrophobically end-capped polyethers are of low molecular weight, they do not efficiently viscosify aqueous systems including latex paints.
Latex compositions containing low molecular weight (3,000-20,000) water-soluble polyethers bearing terminal hydrophobes were disclosed in U.S. Pat. No. 3,770,684. They were claimed to provide improved leveling in latex paints. However, these polyethers were not efficient in increasing the viscosity of water and did not provide other rheological properties demanded in various highly filled aqueous systems. Hence, they were not versatile and economical.
Commercial nonionic synthetic associative thickeners exhibit often poor and variable compatibility in paints, as exemplied by syneresis, poor color acceptance, variable paint viscosity over time, and inadequate hiding power. A need exists in the paint industry for a nonionic synthetic associative thickener that corrects these deficiencies at a cost-effective dosage level. No single thickener is known that provides all the desired performance characteristics required in water-borne coatings. Hence, very often, attempts are made to use blends of two or more different thickeners to achieve the targeted coating rheology. While this approach works in a limited way, blending of thickeners is often cumbersome and depending on mutual interactions between the individual thickener, the stability and performance of the coatings are often in jeopardy.